Desaturable Semiconductor Device with Transistor Cells and Auxiliary Cells

ABSTRACT

A semiconductor device includes transistor cells that connect a first load electrode with a drift structure forming first pn junctions with body zones when a gate voltage applied to a gate electrode exceeds a first threshold voltage. First auxiliary cells in a vertical projection of and electrically connected with the first load electrode are configured to inject charge carriers into the drift structure at least in a forward biased mode of the first pn junctions. Second auxiliary cells are configured to inject charge carriers into the drift structure at high emitter efficiency when in the forward biased mode of the first pn junctions the gate voltage is below a second threshold voltage lower than the first threshold voltage and at low emitter efficiency when the gate voltage exceeds the second threshold voltage.

BACKGROUND

In semiconductor devices including both transistor cells and a diodefunctionality such as MCDs (MOS controlled diodes) and RC-IGBTs (reverseconducting insulated gate bipolar transistors), mobile charge carriersflood a semiconductor region along a forward biased pn junction and forma dense charge carrier plasma resulting in a low forward resistance ofthe diode. When the concerned pn junction commutates thereby changingfrom forward biased to reverse biased, a reverse recovery currentremoves the charge carrier plasma. The reverse recovery currentcontributes to dynamic switching losses of the semiconductor device.Typically, in a desaturation period preceding the change of the pnjunction from forward biased to reverse biased a gated MOS channelattenuates the charge carrier plasma in order to reduce the dynamicswitching losses. A safety period between the end of the desaturationperiod and the beginning of the commutation secures that thesemiconductor device is in a blocking mode with closed MOS channelbefore commutation starts. During the safety period the charge carrierplasma partially recovers and foils to some degree the desaturationmechanism.

It is desirable to improve the switching characteristics ofsemiconductor devices including transistor cells as well as a diodefunctionality.

SUMMARY

According to an embodiment a semiconductor device includes transistorcells configured to connect a first load electrode with a driftstructure forming first pn junctions with body zones when a gate voltageapplied to a gate electrode exceeds a first threshold voltage. Firstauxiliary cells in a vertical projection of and electrically connectedwith the first load electrode are configured to inject charge carriersinto the drift structure at least in a forward biased mode of the firstpn junctions. Second auxiliary cells are configured to inject chargecarriers into the drift structure at high emitter efficiency when in theforward biased mode of the first pn junctions the gate voltage is belowa second threshold voltage lower than the first threshold voltage and atlow emitter efficiency when the gate voltage exceeds the secondthreshold voltage.

According to an embodiment, a semiconductor device includes asemiconductor body that includes a drift structure and cell mesas formedbetween gate structures that extend from a first surface of thesemiconductor body into the drift structure. The cell mesas includebottleneck sections and wide sections between the bottleneck sectionsand the first surface, wherein the wide sections are wider than narrowportions of the bottleneck sections. Transistor cells include body zonesforming first pn junctions with the drift structure and second pnjunctions with source zones. First auxiliary cells are electricallyconnected in parallel to the transistor cells and second auxiliary cellsare electrically connected in parallel to the transistor cells, whereinthe narrow portions of the bottleneck sections in the first auxiliarycells are wider than the narrow portions of the bottleneck sections inthe second auxiliary cells.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description and onviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification. The drawings illustrate the embodiments ofthe present invention and together with the description serve to explainprinciples of the invention. Other embodiments of the invention andintended advantages will be readily appreciated as they become betterunderstood by reference to the following detailed description.

FIG. 1A combines schematic vertical cross-sectional views of portions ofa semiconductor device with transistor cells, first auxiliary cells andsecond auxiliary cells according to an embodiment.

FIG. 1B is a schematic diagram illustrating characteristics of thetransistor cells, first auxiliary cells and second auxiliary cells ofFIG. 1A for discussing effects of the embodiments.

FIG. 1C is a schematic timing diagram illustrating a method of operatingthe semiconductor device of FIG. 1A.

FIG. 2A is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment related to RC-IGBTs.

FIG. 2B is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment related to MCDs. FIG. 3Ais a schematic horizontal cross-sectional view of an RC-IGBT accordingto an embodiment with evenly distributed first auxiliary cells.

FIG. 3B is a schematic horizontal cross-sectional view of an RC-IGBTaccording to an embodiment with first auxiliary cells arranged in thecenter of the diode region.

FIG. 3C is a schematic horizontal cross-sectional view of an RC-IGBTaccording to an embodiment with one or more first auxiliary cellsarranged in a center of a pilot region, which is surrounded by a bipolarregion.

FIG. 3D is a schematic horizontal cross-sectional view of an RC-IGBTaccording to an embodiment with auxiliary cells arranged in peripheralportions of a pilot region, which is surrounded by a bipolar region.

FIG. 4 is a schematic vertical cross-sectional view of an RC-IGBT forillustrating the arrangement of auxiliary cells according to embodimentsrelated to wide collector channels.

FIG. 5A is a schematic horizontal cross-sectional view of a portion ofan RC-IGBT according to an embodiment related to auxiliary cells definedby openings in a barrier structure.

FIG. 5B is a schematic planar projection of a vertical cross-section ofthe semiconductor device portion of FIG. 5A along line B-B.

FIG. 5C is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 5A along line C-C.

FIG. 6A is a schematic vertical cross-sectional view of a portion of anRC-IGBT along a longitudinal mesa axis according to an embodimentrelated to auxiliary cells defined by a barrier structure with locallyattenuated portions.

FIG. 6B is a schematic horizontal cross-sectional view of a portion ofan RC-IGBT according to an embodiment related to auxiliary cells definedby a variation of a cell mesa width.

FIG. 6C is a schematic horizontal cross-sectional view of a portion ofan RC-IGBT according to an embodiment related to auxiliary cells definedin mesas of different widths.

FIG. 7A is a schematic horizontal cross-sectional view of a portion ofan RC-IGBT according to another embodiment related to auxiliary cellsdefined by locally widened cell mesas.

FIG. 7B is a schematic planar projection of a vertical cross-section ofthe semiconductor device portion of FIG. 7A along line B-B.

FIG. 7C is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 7A along line C-C.

FIG. 7D is a schematic horizontal cross-sectional view of a portion ofan RC-IGBT according to another embodiment related to first auxiliarycells in wide cell mesas and second auxiliary cells in narrow cellmesas.

FIG. 8A is a schematic vertical cross-sectional view of a portion of asemiconductor device with injection cells based on cell mesas includingbottleneck sections for illustrating effects of the embodiments.

FIG. 8B is a schematic diagram plotting collector-to-emitter voltage VCEand storage charge QF of the injection cells of FIG. 8A against the gatevoltage for different vertical extensions of narrow portions of thebottleneck sections.

FIG. 8C shows a section of the diagram of FIG. 8B around a gate voltageof OV in detail.

FIG. 9A combines schematic vertical cross-sectional views of portions ofa semiconductor device according to an embodiment related to meta cells.

FIG. 9B is a schematic diagram for illustrating the effect of the metacells in FIG. 9A.

FIG. 10A is a schematic horizontal cross-sectional view of a portion ofa semiconductor device in accordance with an embodiment concerningRC-IGBTs with meta cells.

FIG. 10B is a schematic planar projection of a vertical cross-section ofthe semiconductor device portion of FIG. 10A along line B-B.

FIG. 10C is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 10A along line C-C.

FIG. 11 is a schematic planar projection of a vertical cross-section ofa semiconductor device according to an embodiment combining barrierstructures with bottle shaped gate structures.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and in which are shownby way of illustrations specific embodiments in which the invention maybe practiced. It is to be understood that other embodiments may beutilized and structural or logical changes may be made without departingfrom the scope of the present invention. For example, featuresillustrated or described for one embodiment can be used on or inconjunction with other embodiments to yield yet a further embodiment. Itis intended that the present invention includes such modifications andvariations. The examples are described using specific language, whichshould not be construed as limiting the scope of the appending claims.The drawings are not scaled and are for illustrative purposes only.Corresponding elements are designated by the same reference sign in thedifferent drawings, respectively, if not stated otherwise.

The terms “having”, “containing”, “including”, “comprising” and the likeare open, and the terms indicate the presence of stated structures,elements or features but do not preclude additional elements orfeatures. The articles “a”, “an” and “the” are intended to include theplural as well as the singular, unless the context clearly indicatesotherwise.

The term “electrically connected” describes a permanent low-ohmicconnection between electrically connected elements, for example a directcontact between the concerned elements or a low-ohmic connection via ametal and/or highly doped semiconductor. The term “electrically coupled”includes that one or more intervening element(s) adapted for signaltransmission may be provided between the electrically coupled elements,for example resistors or elements that are controllable to temporarilyprovide a low-ohmic connection in a first state and a high-ohmicelectric decoupling in a second state.

FIG. 1A shows a portion of a semiconductor device 500, for example anMCD such as an MGD (MOS-gated diode) with shorted gate, an RC-IGBT or adevice including an MCD or RC-IGBT functionality. Silicon (Si), siliconcarbide (SiC), germanium (Ge), a silicon germanium crystal (SiGe),gallium nitride (GaN), gallium arsenide (GaAs) or any other A_(III)B_(V)semiconductor forms a semiconductor body 100 of the semiconductor device500.

At a front side the semiconductor body 100 has a first surface 101 whichmay be approximately planar or which may be spanned by coplanar surfacesections. A minimum distance between the first surface 101 and a mainlyplanar second surface at an opposite rear side and parallel to the firstsurface 101 defines the voltage blocking capability of the semiconductordevice 500. For example, the semiconductor body 100 of an RC-IGBTspecified for a blocking voltage of about 1200 V may have a thickness of90 μm to 110 μm. Embodiments related to higher blocking capabilities maybe based on semiconductor bodies 100 with a thickness of several 100 μm.

In a plane perpendicular to the cross-sectional plane, the semiconductorbody 100 may have an approximately rectangular shape with an edge lengthin the range of several millimeters. A normal to the first surface 101defines a vertical direction and directions orthogonal to the verticaldirection are horizontal directions.

The semiconductor body 100 includes a drift structure 120 of a firstconductivity type. The drift structure 120 forms first pn junctions pn1with body zones 115 of the second conductivity type, wherein the bodyzones 115 are formed between the first surface 101 and the driftstructure 120.

Transistor cells TC, first auxiliary cells AC1 and second auxiliarycells AC2 are formed along gate structures 150 extending from the firstsurface 101 down to at least the first pn junctions pn1.

The gate structures 150 include a conductive gate electrode 155 and agate dielectric 151 separating the gate electrode 155 from thesemiconductor body 100. The gate electrode 155 may be a homogeneousstructure or may have a layered structure including one or more metalcontaining layers. According to an embodiment, the gate electrode 155may include or consist of a heavily doped polycrystalline silicon layer.The gate electrode 155 may be electrically connected to a gate connector330 outside the semiconductor body 100. The gate connector 330 may formor may be electrically coupled or connected to a gate terminal G.

The gate dielectric 151 may have a uniform thickness. According to otherembodiments, a bottom portion of the gate dielectric 151 averted fromthe first surface 101 may be thicker than a top portion oriented to thefirst surface 101. The gate dielectric 151 may include or consist of asemiconductor oxide, for example thermally grown or deposited siliconoxide, a semiconductor nitride, for example deposited or thermally grownsilicon nitride, or a semiconductor oxynitride, for example siliconoxynitride.

The transistor cells TC further include source zones 110 of the firstconductivity type forming second pn junctions pn2 with body zones 115assigned to the transistor cells TC. The source zones 110 are formedbetween the first surface 101 and the body zones 115 of the transistorcells TC.

The source zones 110 of the transistor cells TC as well as the bodyzones 115 of the first and second auxiliary cells AC1, AC2 areelectrically connected to a first load electrode 310 which may form orwhich may be electrically coupled or connected to a first load terminalL1. The body zones 115 of the transistor cells TC may also beelectrically connected to the first load electrode 310. The driftstructure 120 is electrically connected to a second load electrode 320which may form or which may be electrically coupled or connected to asecond load terminal L2.

At gate voltages above a first threshold voltage Vthn inversion layersare formed in the body regions 115 of the transistor cells TC as well asin the first and second auxiliary cells AC1, AC2 along the gatedielectric 151. The inversion layers in the transistor cells TC form MOSgated channels for minority charge carriers between the source zones 110and the drift structure 120. The inversion layers in the first andsecond auxiliary cells AC1, AC2 are without connection to the first loadelectrode 310. The first and second auxiliary cells AC1, AC2 differ fromeach other as regards a relationship between a forward voltage acrossthe respective single auxiliary cells AC1, AC2 and the gate voltage whenthe first pn junctions pn1 are forward biased.

The first pn junctions pn1 are forward biased in case of a forwardbiased MCD with a positive voltage applied between the first loadterminal L1 (anode) and the second load terminal L2 (cathode) or, incase of a reverse biased RC-IGBT with a negative voltage applied betweenthe second load terminal L2 (collector) and the first load terminal L1(emitter). The first and second auxiliary cells AC1, AC2 differ fromeach other with respect to their behavior concerning charge carrierinjection efficiency.

The different charge carrier injection characteristics of the first andsecond auxiliary cells AC1, AC2 may result from different emitterefficiencies, wherein emitter efficiency is the ratio of the holecurrent density to the total current density. A variation of emitterefficiency may be achieved by different vertical dopant profiles throughthe first pn junctions pn1, by way of example.

According to another embodiment, the first and second auxiliary cellsAC1, AC2 may have equal or similar emitter efficiencies but differ fromeach other with respect to a width of the body zones 115 and/or thefirst pn junctions pn1 between neighboring gate structures 150. In thefirst auxiliary cells AC1 an injection efficiency given by the integralacross the emitter efficiency from one side of the concerned cell to theopposite side may be greater than the injection efficiency in the secondauxiliary cells AC2. According to a further embodiment, the auxiliarycells AC1, AC2 may have both different emitter efficiencies andhorizontal dimensions.

An inversion channel formed along a gate structure 150 and connectedwith the body zone 115 of an auxiliary cell AC1, AC2 increases theinjection efficiency of the concerned cell such that injectionefficiency at least of the second auxiliary cells AC2 may be controlledby the gate-to-emitter voltage VGE.

During a saturation period the second auxiliary cells AC2 are effectiveas saturation injections cells injecting charge carriers into the driftstructure 120 at a high rate and establishing a dense charge carrierplasma. In a desaturation period the saturation injection cells AC2 areconsiderably less active such that the charge carrier plasma partiallydissipates. For example, during desaturation the mean injectionefficiency of the saturation injection cells AC2 for VGE >Vth2 may be atmost 50%, or at most 10%, or at most 1% of the mean injection efficiencyof the saturation injection cells AC2 for VGE<Vth2.

Instead, during the desaturation period, the first auxiliary cells AC1,which are effective as desaturation injection cells, still injectsufficient charge carriers to maintain a sufficiently low forwardvoltage VF across the desaturation injection cells AC1.

During both the desaturation period and the saturation period, all typesof cells AC1, AC2, TC may inject charge carriers into the driftstructure 120, but in the desaturation period the saturation injectioncells AC2 inject at significantly reduced injection efficiency comparedto the saturation period such that in total less charge carriers areinjected into the drift structure 120.

By providing two different types of injection cells, one type may beadapted to the requirements of the saturation period and the other typecan be adapted to the requirements of the desaturation period. Comparedto approaches with only one type of desaturation cells, processconstrictions can be relaxed and device parameters for the saturationperiod and the desaturation period can be tuned independently from eachother.

FIG. 1B illustrates the different characteristics of the transistorcells TC, the desaturation injections cells AC1 and the saturationinjection cells AC2. The following discussion refers to p-type bodyzones 115 and n-type source zones 110 as, e.g., in an n-channel RC-IGBT.Corresponding considerations apply to semiconductor devices 500 withp-type source zones 110 and n-type body zones 115 as, e.g., in p-channelRC-IGBTs.

According to forward characteristic 701, when a gate voltage VGL1applied between the gate terminal G and the first load terminal L1exceeds the first threshold voltage Vthn, inversion layers formed in thebody zones 115 of the transistor cells TC along the gate dielectrics 151form MOS gated channels that connect the source zones 110 with the driftstructure 120 and that provide an electron path between the first loadelectrode 310 and the drift structure 120. At the same time, due to theabsence of source zones or a missing connection between such sourcezones and the first load electrode 310, inversion layers formed in thebody zones 115 of the desaturation and saturation injection cells AC1,AC2 are without connection to the first load electrode 310. This holdsboth when a positive voltage is applied between the first load terminalL1 and the second load terminal L2 and when a negative voltage isapplied between the first load terminal L1 and the second load terminalL2. The first reverse characteristic 711 illustrates the relationshipbetween a forward voltage VF across single desaturation injection cellsAC1 and the gate voltage VGL1 in a forward biased mode of the first pnjunction pn1 with a negative voltage applied between the second loadterminal L2 and the first load terminal L1. At least for a gate voltageVGL1 below a further threshold voltage Vth0, single desaturationinjection cells AC1 have a low forward voltage VF of at most 0.2%, e.g.,at most 0.15%, of the maximum blocking voltage the semiconductor device500 is specified for. For example, the forward voltage VF is at most 2Vfor a semiconductor device with a blocking capability of 1200V or atmost 5V for a semiconductor device with a blocking capability of 6.5 kV.According to an embodiment, the forward voltage VF is at most 20V at anominal reverse current.

When the gate voltage VGL1 exceeds the further threshold voltage Vth0,the forward voltage VF may increase with increasing VGL1 or may remainapproximately constant up to beyond the first threshold voltage Vthn.According to an embodiment, the injection efficiency of the desaturationinjection cells AC1 sharply decreases and the forward voltage VF acrossthe desaturation injection cells AC1 increases for VGL1>Vth0 to keep theimpact of the desaturation injection cells AC1 on other deviceparameters low. Below the further threshold voltage Vth0 an increase ofthe forward voltage drop across the first desaturation injection cellsAC1 with increasing gate-to-emitter voltage VGE is less steep than abovethe further threshold voltage Vth0.The second reverse characteristic 721illustrates the relationship between a forward voltage VF across singlesaturation injection cells AC2 and the gate voltage VGL1 in the forwardbiased mode of the first pn junction pn1. For a gate voltage VGL1 belowa second threshold voltage Vthp which is lower than the furtherthreshold voltage Vth0, the forward voltage across a single saturationinjection cell AC2 is at most 5V. For VGL1 >Vthp the forward voltagedrop across single saturation injection cells AC2 sharply increases withincreasing VGL1 indicating that the charge carrier injection efficiencyof the saturation injection cells AC2 sharply decreases between Vthp andVth0. Below the second threshold voltage Vthp an increase of the forwardvoltage drop across the saturation injection cells AC2 with increasinggate-to-emitter voltage VGE is less steep than above the furtherthreshold voltage Vthp.

FIG. 1B further shows that an overall charge carrier injectionefficiency ηAC21 of the saturation injection cells AC2 for VGL1<Vthp ishigher than an overall charge carrier injection efficiency ηAC22 of thesaturation injection cells AC2 for VGL1=Vth0, wherein the overall chargecarrier injection efficiency is the surface integral over the localemitter efficiency for the respective cell type. The desaturationinjection cells AC1 may show a lower overall charge carrier injectionefficiency than the saturation injection cells AC2 for VGL1<Vthp butmaintain a sufficient charge carrier injection efficiency at least up toVGL1=Vth0 such that despite of the comparatively low overall chargecarrier injection efficiency the desaturation injection cells AC1 canmaintain a low forward voltage VF for the desaturation period with agate voltage VGL1 between the second threshold voltage Vthp and thefurther threshold voltage Vth0.

Typically, three-level approaches for desaturable RC-IGBTs rely oninjection cells designed with a large spread between a high injectionefficiency at a gate-to-emitter voltage VGE of −15 V on the one hand anda low injection efficiency at VGE=0 V on the other hand. During aninjection period of a reverse conducting mode (RC-mode) of the RC-IGBT,in which the first pn junctions pn1 are forward biased. The injectioncells are active and inject charge carriers into the drift structure 120at high injection efficiency to achieve a dense charge carrier plasma.In a desaturation period preceding commutation, hence before the voltagebetween the first and second load terminals L1, L2 changes polarity, theinjection cells are deactivated and inject charge carriers only at asignificantly lower injection efficiency such that the charge carrierplasma density attenuates.

The desaturation is the more effective the greater the spread of theinjection efficiency is. However, when forming injection cells with highspread between the injection mode and the desaturation mode, it turnsout that the forward voltage VF at VGE=0 may strongly depend on processfluctuations.

Instead, the present embodiments rely on two types of injection cells.The saturation injection cells AC2 may be designed with a high spreadbetween the injection efficiency at VGL1=−15 V and the injectionefficiency at VGL1=0 V such that a high desaturation efficiency can beachieved. The desaturation injection cells AC1 may be designed with noor a low spread between the injection efficiency at VGL1=−15 V and theinjection efficiency at VGL1=0 V but with an injection efficiency thatassures a sufficient low forward voltage VF, e.g. less than 3 V, at andclose to VGL1=0 V, wherein the forward voltage drop VF is less prone toprocess fluctuations. As a result, the embodiments combine highdesaturation efficiency with persistently low forward voltage VF evenduring the desaturation period.

FIG. 1C schematically shows the mode of operation of an RC-IGBT based onthe semiconductor device 500 of FIGS. 1A and 1B.

During a saturation period Sat of the RC-mode of the RC-IGBT, agate-to-emitter voltage VGE is below Vthp and the saturation injectioncells AC2 inject charge carriers at high efficiency into the driftstructure 120 resulting in a high storage charge QF. Thecollector-to-emitter voltage VCE is given by the forward voltage VF1 ofthe reverse diode during the saturation period Sat, wherein thecharacteristics of the reverse diode are governed by the saturationinjection cells AC2. The forward voltage VF1 as well as the forwardresistance Rfwd of the reverse diode is low.

At t=t1, VGE rises to Vthp<VGE<Vth0 and a desaturation period Desatstarts. The saturation injection cells AC2 switch to a low injectionmode. The charge carrier plasma density and the storage charge sharplydecrease whereas forward voltage rises and forward resistance Rfwdincreases. But the desaturation injection cells AC1 are still active andensure an ongoing comparatively low forward voltage VF2 of the reversediode.

After some time a process may be triggered that may change the voltagebias across the RC-IGBT. For example, a semiconductor switch in theother part of a half bridge is turned-on while the RC-IGBT remainsturned off. During turn-off the voltage bias of the RC-IGBT mayrepeatedly change from reverse biased to forward biased and vice versa.Finally, the voltage bias may change to forward biased whereby theRC-IGBT commutates and may directly change into a blocking state.

Since the charge carrier plasma density has been reduced, a smallernumber of charge carriers have to be drained off from the semiconductorbody 100 than without desaturation period Desat and switching losses arereduced. Since desaturation gets along without formation of any MOSgated channel, the RC-IGBT can immediately sustain the full blockingvoltage in a blocking phase Blk of the forward biased state. When thecommutation charge carrier flow ends, the gate voltage may be againlowered to below the second threshold voltage in order to improverobustness against strong current filaments in the wake of thecommutation charge carrier flow and to avoid a dynamic increase of thegate potential above Vthn.

Later, e.g., at t=t3, VGE may rise to above Vthn, the MOS gated channelsin the transistor cells TC turn on and the RC-IGBT changes to aconducting phase Cnd of the forward biased state.

The first, second and further threshold voltages Vthn, Vthp, Vth0 areselected to meet worst case conditions specified for the gate voltagelevels. For example, Vth0 may be selected such that the forward voltageof the desaturation injection cells AC1 is below 15V for a 1200V deviceover the whole admissible gate voltage range for the desaturation mode.The second threshold voltage Vthp may be selected such that the secondauxiliary cells AC2 contribute to overall hole injection only to a lowdegree, e.g., at most 30%, or at most 15%, or at most 5% over thecomplete admissible gate voltage range for the desaturation mode.

For example, datasheets of depletable three-level RC-IGBTs may specifygate voltage levels of +10 to +25 V for switching on the MOS-gatedchannels, −3 V to +3 V for the desaturation mode and −15 to −25 V forthe saturation mode. For an RC-IGBT with the above specifications, thefirst threshold voltage Vthn may be about half way between +1 V and +10V, e.g. close to 5.5 V. The further threshold voltage Vth0 may bebetween the maximum admissible voltage level for the desaturation modeand the first threshold voltage Vthn, hence in a range between +1.0 Vand +5.0 V, for example between 2 V and 3 V. The second thresholdvoltage Vthp may be half way between −15 V and the minimum admissiblevoltage level for the desaturation mode, hence in a range between −10 Vand −1 V, for example at −5.5 V.

FIG. 2A illustrates an RC-IGBT or a semiconductor device including anRC-IGBT 501 according to an embodiment. At a front side of a siliconsemiconductor body 100 including a drift structure 120 as describedabove a transistor module 601 includes controllable cells Ce, which maybe transistor cells TC, desaturation injection cells AC1 or saturationinjection cells AC2 as described above. Control electrodes of thecontrollable cells CE are electrically connected to a gate conductor330, which may form or which may be electrically connected or coupled toa gate terminal G. Source and body zones of the controllable cells CEare electrically connected to a first load electrode 310, which may formor which may be electrically connected or coupled to an emitter terminalE.

The drift structure 120 includes a drift zone 121 and may directlyadjoin to the body zones in the controllable cells Ce. According toother embodiments, a heavier doped barrier layer may be sandwichedbetween the body zones 115 and the drift zone 121. In the drift zone121, a dopant concentration may gradually or in steps increase ordecrease with increasing distance to the first surface 101 at least inportions of its vertical extension. According to other embodiments, thedopant concentration may be approximately uniform in the complete driftzone 121. A mean dopant concentration in the drift zone 121 may bebetween 1E12 cm⁻³ and 1E15 cm⁻³, for example in a range from 5E12 cm⁻³to 5E13 cm⁻³.

A pedestal layer 130 formed along a second surface 102 at an oppositerear side directly adjoins a second load electrode 320 which may form orwhich may be electrically connected to a collector terminal C. Thepedestal layer 130 includes first zones 131 of the conductivity type ofthe body zones 115 and second zones 132 of the conductivity type of thedrift structure 120. The first and second zones 131, 132 extend from thedrift structure 120 to the second load electrode 320, respectively. Thefirst zones 131 are effective as rear side emitter zones injectingminority charge carriers into the drift structure 120 in a conductingphase of the forward biased state. The second zones 132 are effective ascollector shorts bypassing the rear side emitter zones in the RC-mode.The first zones 131 may alternate with the second zones 132 in a bimodalregion 620 of the RC-IGBT 501. In addition to the bimodal region 620,the RC-IGBT 501 may include a pilot region 610 with a pilot zone 133,wherein a horizontal extension of the pilot zone 133 is at least twice,e.g. at least ten times as large as a horizontal extension of the firstzones 131 in the bimodal region 620. The pilot zone 133 supportsignition of the conducting phase of the forward biased state.

The dopant concentrations in the first and second zones 131, 132 and, ifapplicable, in the pilot zone 133, are sufficiently high to ensure a lowohmic contact to the second load electrode 320. For example, a maximumdopant concentration along the second surface 102 in p doped first,second or pilot zones 131, 132, 133 may be at least 1E16 cm⁻³, forexample at least 5E17 cm⁻³. A maximum dopant concentration in n-dopedfirst, second or pilot zones 131, 132, 133 may be at least 1E18 cm⁻³.

The drift structure 120 may include a field stop layer 128 of theconductivity type of the drift zone 121, wherein the field stop layer128 separates the drift zone 121 from the pedestal layer 130. A mean netdopant concentration in the field stop layer 128 is at least twice ashigh as a maximum net dopant concentration in the drift zone 121 and atmost half as high as the mean dopant concentration in the second zones132 of the pedestal layer 130. The drift structure 120 may includefurther doped zones, for example zones forming a compensation structure,barrier zones for locally increasing a charge carrier plasma densityand/or buffer zones locally shaping the electric field.

FIG. 2B refers to an MCD 502 or a semiconductor device comprising an MCD502. The source and body zones of the controllable cells Ce may beelectrically connected to the first load electrode 310, which may formor which may be electrically connected to an anode terminal A. The gateelectrodes of the controllable cells Ce may be electrically connected agate terminal or to the first load electrode 310. The pedestal layer 130is a heavily doped layer of the conductivity type of the drift zone 121,wherein a maximum net dopant concentration in the pedestal layer 130along the second surface 102 ensures a low ohmic contact to the secondload electrode 320, which forms or which is electrically connected to acathode terminal K. For further details reference is made to thedescription of the RC-IGBT 501 of FIG. 2A.

The total area ratio of desaturation injection cells AC1 to saturationinjection cells AC2 may range from 1:1 to 1:10000, e.g., from 1:20 to1:500. The total area ratio of transistor cells TC to injection cellsAC1, AC2 may be in a range from 200:1 to 1:50, e.g., from 10:1 to 1:10.Placement of the saturation and desaturation injection cells AC2, AC1may be unrelated to the position of the first, second and pilot zones131, 132, 133 at a rear side.

In FIG. 3A, the RC-IGBT 501 includes a bipolar region 620 andcomparatively small first auxiliary cells AC1 are evenly distributedwithin the complete bipolar region 620. Evenly distributed desaturationinjection cells AC1 avoid high local current densities duringcommutation. An avalanche induced portion of a reverse recovery chargeQrr can be kept small. According to other embodiments, the saturationand desaturation injection cells AC2, AC1 at the front side are alignedto the pattern of the first, second and pilot zones 131, 132, 133 at therear side.

In FIG. 3B, the desaturation injection cells AC1 are placed in a bimodalregion 620 surrounding a pilot region 610. In a portion of thesemiconductor body 100 oriented to the rear side, the reverse currentmainly flows in the bimodal region 620 during the saturation period ofthe RC-mode. By placing the desaturation injection cells AC1 only in thebimodal region 620, the reverse current nearly exclusively flows in thebimodal region 620 over the complete vertical extension of thesemiconductor body 100 shortly before commutation. Injecting, during thedesaturation period, charge carriers exclusively in the bimodal region620 results in that the charge carrier plasma is concentrated in thebimodal region 620. As a consequence, a commutation charge carrier flowhas no or only a weak horizontal component and does not generate asufficient horizontal voltage drop to ignite charge carrier injectionfrom the pilot region 610. The pilot zone 133 is prevented from unwantedignition.

As illustrated in FIG. 3B a contiguous desaturation injection cell AC1may surround the pilot region 610, wherein the desaturation injectioncell AC1 may be centered to the bimodal region 620 that surrounds thepilot region 610. A resulting reverse current gain of the pilot region610 is low. According to other embodiments, a plurality of isolateddesaturation injection cells AC1 may be arranged in a stripe centered tothe bimodal region 620 and surrounding the pilot region 610.

FIGS. 3C to 3D show RC-IGBTs 501 with the desaturation injection cellsAC1 formed in the pilot region 610 in the vertical projection of thepilot zone 133. The desaturation injection cells AC1 locally increasethe reverse current gain of the pilot region 610 and in this way mayimprove switching softness of the RC-IGBT 501. Minority charge carriersthat have been previously injected through the desaturation injectioncells AC1, e.g., holes in case of an n-channel RC-IGBT 501, may ignitethe pilot zone 133 during commutation such that a bipolar transistorformed by the body zones 115, the drift structure 120 and the pilot zone133 turns on. Ignition of the pilot zone 133 may improve softness of theswitching behavior and robustness against oscillations at costs ofincreased switching losses.

In the RC-IGBT 501 of FIG. 3C one or a small number of desaturationinjection cells AC1 is/are placed in the center of the pilot zone 133 toincrease the horizontal path lengths of charge carriers previouslyinjected by the desaturation injection cells AC1.

In FIG. 3D, the first desaturation injection cells AC1 are arrangedsymmetrically in the edges of the pilot region 610 and close to thebimodal region 620 to achieve a tradeoff between low switching lossesand high switching softness.

FIG. 4 shows possible positions of desaturation injection cells AC11,AC12, AC13 in an RC-IGBT 501 with expanded second zones 132 providingcollector shorts for the RC-mode. According to an embodiment with thedesaturation injection cells AC11 placed in the vertical projection ofthe second zones 132 and at a large horizontal distance z1 to theadjoining first zone 131, a reverse current through the semiconductorbody 100 during desaturation mainly flows in a vertical directionbetween the second zones 132 and the desaturation injection cells AC11.Charge carrier plasma density remains low in the vertical projection ofthe first zones 131. When the RC-IGBT commutates, the resultingcommutation charge carrier flow has no or only a weak horizontalcomponent, generates no or only a low horizontal voltage drop along thepn junctions between the first zones 131 and the drift structure 120 anddoes not ignite the bipolar transistor formed by the body zones 115 ofthe controllable cells CE, the drift structure 120 and the first zones131.

By placing the desaturation injection cells AC12, AC13 at a smallhorizontal distance z2 to the first zones 131 in the vertical projectionof the second zones 132 and/or at a horizontal distance z3 to the secondzones 132 in the vertical projection of the first zones 131 results inan increased charge carrier plasma density in the vertical projection ofthe first zones 131, such that a charge carrier flow during commutationhas a horizontal component. The resulting horizontal voltage drop maytrigger ignition of the bipolar transistor such that switching softnesscan be improved, if applicable at costs of increased switching losses.Placement of the desaturation injection cells AC11, AC12, AC13 withrespect to the second zones 132 forming the collector shorts, as well asnumber and lateral extension of the desaturation injection cells AC1determines the conditions at which the commutation charge carrier flowtriggers charge carrier injection from the first zones 131.

The desaturation injection cells AC1 differ from the saturationinjection cells AC2 in that the desaturation injection cells AC1 have ahigher threshold voltage up to which they inject charge carriers at highefficiency. This effect can be achieved by a variation of geometricaldimensions and/or dopant gradients in the injection cells AC1, AC2, byway of example.

FIGS. 5A to 5C refer to RC-IGBTs 501 or other semiconductor devicesincluding an RC-IGBT 501 with desaturation injection cells AC1 formed bya local variation of a barrier structure 125 between the body zones 115of the injection cells AC1, AC2 and the drift zone 121. The RC-IGBT 501is based on a semiconductor body 100 as described in detail with regardto FIGS. 1A to 1C, wherein the semiconductor body 100 includes a driftstructure 120 of a first conductivity type, a body zone 115 of a second,opposite conductivity type between the first surface 101 and the driftstructure 120 as well as a pedestal layer 130 sandwiched between thedrift structure 120 and the second surface 102.

For the illustrated n-channel RC-IGBT 501, the first conductivity typeis n-type and the second conductivity type is p-type. Similarconsiderations as outlined below apply to p-channel RC-IGBTs with thefirst conductivity type being p-type and the second conductivity typebeing n-type.

The drift structure 120 includes a drift zone 121 with a dopantconcentration that may gradually or in steps increase or decrease withincreasing distance to the first surface 101 at least in portions of itsvertical extension. According to other embodiments the dopantconcentration in the drift zone 121 may be approximately uniform. For anRC-IGBT 501 based on silicon, a mean dopant concentration in the driftzone 121 may be between 1E12 cm⁻³ and 1E15 cm⁻³, for example in a rangefrom 5E12 cm⁻³ to 1E14 cm⁻³. In case of an RC-IGBT 501 based on SiC, amean dopant concentration in the drift zone 121 may be between 5E14 cm⁻³and 1E17 cm⁻³, for example in a range from 1E15 cm⁻³ to 1E16 cm³.

The pedestal layer 130 includes first zones 131 of the conductivity typeof the body zones 115 and second zones 132 of the conductivity type ofthe drift zone 121. The first zones 131 are effective as rear sideemitter zones injecting minority charge carriers into the drift zone 121in the conducting phase. The second zones 132 form collector shortsbypassing the first zones 131 in the RC-mode. Impurity concentrations inthe first and second zones 131, 132 are sufficiently high for forming anohmic contact with a metal directly adjoining the second surface 102. Amean dopant concentration for p-type zones may be at least 1E16 cm⁻³,for example 5E17 cm⁻³, and a mean dopant concentration for n-type zonesmay be at least 1E18 cm⁻³, for example at least 5E19 cm⁻³.

The drift structure 120 may include a field stop layer 128 of theconductivity type of the drift zone 121. The field stop layer 128separates the pedestal layer 130 from the drift zone 121, wherein a meandopant concentration in the field stop layer 128 may be lower than themean dopant concentration in the second zones 132 of the pedestal layer130 by at least 50%, e.g., by at least one order of magnitude and may behigher than in the drift zone 121 by at least 100%, e.g. by at least oneorder of magnitude.

The first and second zones 131, 132 of the pedestal layer 130 extendfrom the second surface 102 to the field stop layer 128 or, in absenceof a field stop layer, to the drift zone 121, respectively. The firstzones 131 may be dots horizontally embedded by second zones 132 forminga grid or vice versa. According to other embodiments, the first andsecond zones 131, 132 may be stripes running parallel to a firsthorizontal direction or may form nested rectangular frames, by way ofexample. Control structures 150 of transistor cells TC, saturationinjection cells AC2 and desaturation injection cells AC1 extend from thefirst surface 101 into the drift zone 121. Portions of the semiconductorbody 100 between neighboring control structures 150 form cell mesas 170.

The control structures 150 may be stripes extending along an extensiondirection of the cell mesas 170. According to an embodiment theextension direction may be exclusively parallel to a first horizontaldirection such that the cell mesas 170 and the control structures 150are straight stripe structures. According to another embodiment, theextension direction alters with respect to the first horizontaldirection such that the cell mesas 170 and the control structures 150form staggered stripes.

The cell mesas 170 may be regularly arranged at a uniformcenter-to-center distance of, for example 400 nm to 20 μm, for example800 nm to 2 μm. A distance between the first surface 101 and the bottomof the control structures 150 may range from 1 μm to 30 μm, e.g., from 2μm to 6 μm. A lateral width of the cell mesas 170 may range from 0.05 μmto 10 μm, e.g., from 0.1 μm to 1 μm.

The control structures 150 include a gate electrode 155 and a gatedielectric 151 separating the gate electrode 155 from the semiconductorbody 100. The gate electrode 155 may be a homogenous structure or mayhave a layered structure including one or more conductive layers.According to an embodiment the gate electrode 155 may include or consistof heavily doped polycrystalline silicon. The gate electrodes 155 may beelectrically connected to a gate terminal G.

The gate dielectric 151 may include or consist of a semiconductor oxide,for example thermally grown or deposited silicon oxide, a semiconductornitride, for example deposited or thermally grown silicon nitride, or asemiconductor oxynitride, for example silicon oxynitride.

Transistor cells TC, saturation injections cells AC2 and desaturationinjections cells AC1 may be directly concatenated to each other along ahorizontal direction.

FIG. 5A shows transistor cells TC, saturation injection cells AC2 anddesaturation injection cells AC1 directly concatenated along a firsthorizontal direction defined by the longitudinal axes of the controlstructures 150. Transistor cells TC, saturation injection cells AC2 anddesaturation injection cells AC1 may directly adjoin to each other,wherein transitions between the different cell types may be gradual orabrupt. According to other embodiments, the transistor cells TC and thedesaturation injection cells AC1 are formed along different controlstructures 150 running parallel to each other.

The body zones 115 are formed in first sections of the cell mesas 170oriented to the first surface 101 and may directly adjoin the firstsurface 101 in the saturation and desaturation injections cells AC2,AC1. A mean net impurity concentration in the body zones 115 may be inthe range from 1E16 cm⁻³ to 5E18 cm⁻³, for example between 1E17 cm⁻³ and5E17 cm⁻³. Each body zone 115 may form a first pn junction pn1 with thedrift structure 120.

Portions of the cell mesas 170 assigned to the transistor cells TCinclude source zones 110 forming second pn junctions pn2 with the bodyzones 115 of the transistor cells TC. Portions of the cell mesas 170assigned to the saturation and desaturation injections cells AC2, AC1may be devoid of any source zone or include source zones withoutconnection to the first load electrode 310.

The source zones 110 may be formed as wells extending from the firstsurface 101 into the body zones 115 and define the transistor cells TCwhich are arranged along a longitudinal horizontal axis of therespective cell mesa 170. Shadowed regions without source zones 110separate neighboring transistor cells TC assigned to the same cell mesa170, wherein in the shadowed regions the body zones 115 of thesaturation and desaturation injections cells AC2, AC1 directly adjointhe first surface 101. Transistor cells TC and shadowed regionsalternate along the longitudinal axis of the respective cell mesa 170.

A distance between neighboring source zones 110 arranged along thelongitudinal axis may be in a range from 1 μm to 200 μm, for example ina range from 3 μm to 100 μm.

A dielectric structure 200 may separate the first load electrode 310from the first surface 101. The dielectric structure 200 may include oneor more dielectric layers from silicon oxide, silicon nitride, siliconoxynitride, doped or undoped silicon glass, for example BSG (boronsilicate glass), PSG (phosphorus silicate glass), or BPSG (boronphosphorus silicate glass), by way of example.

The first load electrode 310 may form an emitter terminal E or may beelectrically coupled or connected to an emitter terminal E of theRC-IGBT 501.

Contact structures 315 extend from the first load electrode 310 throughthe dielectric structure 200 into the semiconductor body 100. Thecontact structures 315 electrically connect the first load electrode 310with the source zones 110 and the body zones 115. A plurality ofspatially separated contact structures 315 may directly adjoin therespective cell mesa 170, wherein at least some of the contactstructures 315 may be assigned to the source zones 110. Otherembodiments may provide stripe-shaped contact structures 315 that extendalong the whole longitudinal extension of the respective cell mesa 170and that directly adjoin the body zones 115 in the shadowed regions.

A second load electrode 320 directly adjoins the second surface 102 andthe pedestal layer 130. The second load electrode 320 may form or may beelectrically connected to a collector terminal C.

Each of the first and second load electrodes 310, 320 may consist of orcontain as main constituent(s) aluminum (Al), copper (Cu), or alloys ofaluminum or copper, for example AlSi, AlCu or AlSiCu. According to otherembodiments, at least one of the first and second load electrodes 310,320 may contain as main constituent(s) nickel (Ni), titanium (Ti),tungsten (W), tantalum (Ta), silver (Ag), gold (Au), platinum (Pt),and/or palladium (Pd). For example, at least one of the first and secondload electrodes 310, 320 may include two or more sub-layers, whereineach sub-layer contains one or more of Ni, Ti, Ag, Au, Pt, W, and Pd asmain constituent(s), e.g., a silicide, a nitride and/or an alloy.

The cell mesas 170 further include a barrier structure 125 which may besandwiched between the body zones 115 and the drift zone 121 such thatthe barrier structure 125 forms the first pn junctions pn1 with the bodyzones 115 and unipolar homojunctions with the drift zone 121. Thebarrier structure 125 has the same conductivity type as the drift zone121. A mean dopant concentration in the barrier structure 125 is atleast ten times as high as the mean dopant concentration in the driftzone 121. According to an embodiment, the mean dopant concentration inthe barrier structure 125 may range from 1E16 cm⁻³ to 1E18 cm⁻³, forexample from 1E17 cm⁻³ to 5E17 cm⁻³. The impurities in the barrierstructure 125 may be phosphorous (P), arsenic (As), selenium (Se) and/orsulfur (S) atoms/ions in case of an n-channel IGBT 501.

According to other embodiments, the barrier structure 125 may beembedded within the body zones 115 such that portions of the body zones115 separate the barrier structure 125 from the drift zone 121.According to further embodiments, the barrier structure 125 is formedwithin the drift zone 121 at a distance to the first pn junctions pn1.

In the conducting phase of the forward biased state, the barrierstructure 125 forms a barrier for charge carriers to escape from thecharge carrier plasma and increases charge carrier plasma density.Further, in the more heavily doped barrier structure 125 minority chargecarriers recombine at a higher rate such that the barrier structure 125reduces minority charge carrier emitter efficiency with respect to thedrift zone 121. The barrier structure 125 is formed at least in thesaturation injection cells AC2 and may also be formed in the transistorcells TC.

FIG. 5C shows gaps 125 a in the barrier structure 125. The gaps 125 alocally increase emitter efficiency with respect to the drift zone 121and define the desaturation injection cells AC1.

The n-channel RC-IGBT 501 of FIG. 6A shows local attenuated portions 125b of the barrier structure 125 defining the desaturation injection cellsAC1. In the desaturation injection cells AC1, the mean dopantconcentration/dose in the attenuated portions 125 b is at most 50%, forexample at most 10% of the dopant concentration/dose in portions of thebarrier structure 125 outside the attenuated portions 125 b in thedesaturation injection cells AC1.

In FIG. 6B an RC-IGBT 501 includes saturation injections cells AC2 innarrow portions of cell mesas 170, wherein the narrow portions have anarrow mesa width y1, and desaturation injection cells AC1 in wideportions of cell mesas 170, wherein the wide portions have a wide mesawidth y2. In the desaturation injection cells AC1 the local injectionefficiency per cell length unit is higher than in the saturationinjection cells AC2. But since the total area assigned to saturationinjection cells AC2 is greater than the total area assigned todesaturation injection cells. AC1, for VGE<Vthp the total injectionthrough the saturation injection cells AC2 may exceed the totalinjection through the desaturation injection cells AC1.

In FIG. 6C an RC-IGBT 501 includes saturation injections cells AC2 innarrow cell mesas 170 x with a narrow mesa width y1 and desaturationinjection cells AC1 in wide cell mesas 170 y with a wide mesa width y2.According to an embodiment, the wide mesa width y2 may be in a rangefrom 100 nm to 20 μm, e.g., in a range from 300 nm to 1000 nm or from400 nm to 800 nm, whereas the narrow mesa width y1 may be in a rangefrom 10 to 400 nm, e.g., in a range from 50 to 200 nm and wherein thewide mesa width y2 is at least 90 nm greater than the narrow mesa widthy1.

The transistor cells TC may be formed in the narrow cell mesas 170 x, inthe wide cell mesas 170 y, or in both of them. Though in thedesaturation injection cells AC1 the local injection efficiency per celllength unit may be higher than in the saturation injection cells AC2,for VGE <Vthp the total injection through the saturation injection cellsAC2 may exceed the total injection through the desaturation injectioncells AC1 if the total area assigned to saturation injection cells AC2is sufficiently great with respect to the total area assigned to thedesaturation injection cells AC1.

The embodiments of FIGS. 6A to 6C may be combined with each other. Forexample, the RC-IGBT 501 of FIG. 6C may include a patterned barrierstructure 125 as illustrated in FIG. 6A or a non-patterned barrierstructure 125 or does not include any barrier structure.

FIGS. 7A to 7C refer to n-channel RC-IGBTs 501 with bottle-shapedcontrol structures 150. The control structures 150 include bulgedsections 150 a and narrow sections 150 b between the bulged sections 150a and the first surface 101, wherein the narrow sections 150 b extendfrom the first surface 101 down to at least the first pn junction pn1.The narrow sections 150 b have a width wc1. In the bulged sections 150 athe control structures 150 have a maximum width wc2 which is greaterthan the first width wc1. The maximum width wc2 is at least 50 nm, forexample at least 100 nm greater than the width wc1 of the narrowsections 150 b.

Accordingly, the cell mesas 170 have wide sections 170 b sandwichedbetween neighboring narrow sections 150 b of the control structures 150and bottleneck sections 170 a sandwiched between neighboring bulgedsections 150 a of the control structures 150. The wide sections 170 binclude at least the body zones 115 and the first pn junctions pn1 aswell as portions of the drift zone 121. A mesa width wm1 a of widesections 170 b in portions of the cell mesas 170 assigned to transistorcells TC and saturation injection cells AC2 is in a range from 100 nm to900 nm, for example in a range from 300 nm to 800 nm. A minimum mesawidth wm2 a of bottleneck sections 170 a in portions of cell mesas 170assigned to transistor cells TC and saturation injection cells AC2 maybe in a range from 10 nm to 400 nm, for example in a range from 50 nm to200 nm. The bottleneck sections 170 a include portions of the drift zone121 and may include portions of a barrier structure, respectively.

Mesa widths of the cell mesas 170 in the saturation injection cells AC2may be the same as in the transistor cells TC. Portions of the cellmesas 170 assigned to the desaturation injection cells AC1 may be widerthan portions of cell mesas 170 assigned to the saturation injectioncells AC2. For example, a width wm2 b of narrow portions of thebottleneck sections 170 a of portions of cell mesas 170 in thedesaturation injection cells AC1 is at least 10% greater, for example atleast 30% greater than a width wm2 a of narrow portions of thebottleneck sections 170 a of portions of cell mesas 170 in thesaturation injection cells AC2. For example, the width wm2 b may be atleast 50 nm, e.g., at least 150 nm greater than the width wm2 a.

In the following, reference is made to the definition of thresholdvoltages in FIG. 1B. When a gate voltage VGE lower than the secondthreshold voltage Vthp is applied to the gate terminal G, p-typeinversion layers are formed in the drift zone 121 around the controlstructures 150. The inversion layers are connected to the body zones 115which in turn are connected to the first load electrode 310 such thatthe inversion layers in the drift zone 121 are effective as chargecarrier emitters. The injected charge carriers increase charge carrierplasma density in the drift zone 121. A high charge carrier plasmadensity results in low forward resistance and low forward voltage of thereverse diode in the RC-mode of the RC-IGBT 501 during a saturationperiod. Both the bulgy form of the gate structures 150 and a barrierstructure as illustrated in FIGS. 5A to 5C contribute to increasing thespread between the injection efficiencies at VGE<Vthp and at VGE>Vthp.The higher the spread is the better is the desaturation efficiency inthe desaturation period.

During a desaturation period of the RC-mode preceding a commutation ofthe RC-IGBT 501, the gate voltage VGE is raised to a voltage greaterthan the second threshold voltage Vthp but lower than the furtherthreshold voltage Vth0. The inversion channels dissipate. In thesaturation injection cells AC2 the bulged sections 150 a of the controlstructures 150 shield the body zones 115 against a contiguous portion ofthe drift structure 120 between the control structures 150 and thepedestal layer 130. The remaining charge carrier injection efficiency ofthe body zones 115 in the saturation injection cells AC2 is low.

Instead, in the desaturation injection cells AC1, the body zones 115preserve a comparatively high injection rate through the widerbottleneck portions 170 a of the cell mesas 170 despite the absence ofp-type inversion layers along the control structures 150. The overallinjection efficiency of the desaturation injection cells AC1 remainssufficiently high to ensure a sufficient charge carrier plasma densityin the drift zone 121 during the desaturation period and a sufficientlylow forward voltage drop across the reverse diode even in thedesaturation period of the RC-mode. Due to the less critical mesadimensions in the desaturation injection cells AC2, the forward voltagedrop governed by the desaturation injection cells AC2 is lesssusceptible to dimensional variations and process fluctuations.

The RC-IGBT 501 combines a high spread of the overall injectionefficiency and, as a consequence high desaturation efficiency, with asufficient minimum injection efficiency in the desaturation period, and,as a consequence a stable forward voltage behavior in the RC-mode.

FIG. 7D refers to a layout with the desaturation injection cells AC1formed in wide cell mesas 170 y that have a mesa width wm1 b greaterthan a mesa width wm1 a of narrow cell mesas 170 x including thesaturation injection cells AC2 or the saturation injection cells AC2 andthe transistor cells TC. The cross-section along line B-B may correspondto the cross-section along line B-B in FIG. 7A with the mesa widths wm1a and wm1 b referring to the widths of the wide mesa sections ofbottleneck mesas. The wide cell mesas 170 y may include source zones 110or may be devoid of source zones 110. Transistor cells TC and saturationinjection cells AC1 may be formed in the same narrow cell mesas 170 x orin different narrow cell mesas 170 y.

According to another embodiment, the drift structure 120 includesbarrier structures as illustrated in FIGS. 5A to 5C and the sidewalls ofthe wide and narrow cell mesas 170 y, 170 x may be approximatelyvertical.

FIGS. 8A to 8C illustrate the correlation of the forward voltage VF ofthe reverse diode in the RC-mode, the storage charge QF in the driftzone 121 and the vertical extension of a narrow portion in thebottleneck section of the cell mesas 170.

FIG. 8A shows injection cells AC which control structures 150 have atotal vertical extension of about 5 μm. The control structures 150 arebottle-shaped with a minimum width wc1 in a narrow section 150 b closeto the first surface 101 and a maximum width wc2 in a bulged section 150a in a distance to the first surface 101. The minimum width wc1 is about1 μm and the maximum width wc2 is about 1.2 μm.

The cell mesas 170 include bottleneck sections 170 a with a width wm2 ofabout 200 nm in a narrow portion of approximately constant width andwide sections 170 b between the first surface 101 and the bottlenecksections 170 a with a width wm1 of about 400 nm. The wide sections 170 binclude the body zones 115. A width of the first pn junctions pn1approximates the width wm1 of the wide sections 170 b. The verticalextension of the narrow portions of the bottleneck sections 170 a may bein a range from 300 nm to 4 μm.

In FIG. 8B the contiguous lines 801-804 show the collector-to-emittervoltage VCE in the RC-mode of an RC-IGBT including the injection cellsAC of FIG. 8A as a function of the gate voltage VGE at verticalextensions of the narrow portions of the bottleneck sections 170 a of2.5 μm (801), 2.1 μm (802), 1.8 μm (803) and 1.5 μm (804). The dottedlines 811, 812, 813, 814 show the storage charge QF, which isproportional to the injection efficiency, at a vertical extension of thenarrow portions of the bottleneck sections of the cell mesas 170 of 2.5μm (811), 2.1 μm (812), 1.8 μm (813) and 1.5 μm (814). The amount ofcurrent that flows through the structure does not depend on the gatevoltage VGE and is in the range of the nominal value.

FIG. 8C shows a portion of the diagram of FIG. 8B around VGE=0 in moredetail.

Both the collector-to-emitter voltage VCE and the storage charge QFstrongly depend on the gate voltage VGE and strongly vary at and aroundVG=0 V. The collector-to-emitter voltage gradient 804 assigned to avertical extension of the narrow portion of 1.5 μm ensures a low forwardvoltage drop for the respective injection cell at VG=0, which is thetypical gate voltage level for the desaturation mode of a three-leveldesaturable RC-IGBT. On the other hand, process fluctuations may resultin that the vertical extension of the narrow portions of the bottlenecksections is smaller than 1.8 μm, resulting in the collector-to-emittervoltage gradient 802 and a forward voltage VF of the concerned injectioncells AC of more than 100 V.

The embodiments allow for combining injection cells AC with the chargestorage gradient 811 ensuring a high spread of the charge storagebetween VGE=−15V and VGE=0V with injection cells having acollector-to-emitter voltage gradient similar to thecollector-to-emitter voltage gradient 804 ensuring a low voltage dropeven at a desaturation gate voltage VG=0V.

FIGS. 9A to 9B refer to embodiments including, in addition to the firstand second auxiliary cells AC1, AC2 third auxiliary cells AC3 (metacells) for maintaining a sufficient degree of charge carrier injectioneven at a gate voltage VGL1 that exceeds the first threshold voltageVthn, which is the threshold voltage for the MOS gated channels throughthe body zones 115 in the transistor cells TC.

The meta cells AC3 may be evenly distributed among the transistor cellsTC, the saturation injection cells AC2 and the desaturation injectioncells AC1. The meta cells AC3 are designed such that they have asufficiently high charge carrier injection efficiency even at gatevoltage levels above the first threshold voltage Vthn. In an n-channelRC-IGBT, the meta cells AC3 are effective as hole emitters even if apositive gate voltage VGE induces inversion channels through the bodyzones 115 of the transistor cells TC.

In a typical three-level operation mode for desaturable n-channelRC-IGBTs, a negative gate voltage VGE is used to increase the holeinjection efficiency in injection cells. A gate voltage VGE of about 0 Vis applied for a desaturation period in which injection efficiency ofthe injection cells is reduced.

On the other hand, typical applications of RC-IGBTs include driver unitscontrolling the gate voltage of the RC-IGBT and sensing a current,wherein the driver unit may turn the RC-IGBT on when the sensed currentis below a certain threshold, which may be about 10% of a nominalcollector current I_(C,nom) the RC-IGBT is specified for. Since for lowcurrents the driver units typically do not always reliably detect theactual current direction, the driver unit may turn on the RC-IGBT evenif the RC-IGBT is reversed biased. As a result, the driver unit mayapply a gate voltage VGE of +15 V to the gate terminal of the RC-IGBTeven in the RC-mode. For this case, the meta cells AC3 may ensure thatsufficient holes are injected into the drift structure 120 to maintain asufficiently dense charge carrier plasma and to avoid a runaway of thevoltage drop across the reverse diode in the RC-mode. The meta cells AC3inject sufficient charge carriers for maintaining a bipolar current atgate voltages above the first threshold voltage Vthn.

The three types of auxiliary cells including saturation injection cellsAC2, desaturation injection cells AC1 and meta cells AC3 allow foradapting the modes of operation below Vthp, between Vthp and Vth0 andabove Vth0 in the RC-mode independently from each other. The meta cellsAC3 may be arranged exclusively in the bipolar regions, e.g.,exclusively in the vertical projection of collector channels 132. Metacells AC3 may have a reduced anode efficiency compared to the injectioncells AC1. The meta cells AC3 may be defined by local dimensionvariations of the body zones 115 or by a variation of the verticaldopant profiles in the cell mesas 170.

According to an embodiment, the body zones 115 in the meta cells AC3 mayhave a lower dopant dose/concentration than the body zones 115 of thesaturation and desaturation injection cells AC2, AC1. According toanother embodiment, a dopant concentration in the drift zone 121 alongthe first pn junctions pn1 is increased and may form a local barrierstructure 125 or a locally enhanced portion 125 c of a barrier structure125 extending along the first pn junctions pn1. According to otherembodiments, an increased concentration of recombination centers mayreduce the effective dopant concentration in the body zones 115. Anotherembodiment may increase the width of the cell mesas 170 in the metacells AC3.

Transitions between the meta cells AC3 and adjoining transistor cellsTC, saturation injection cells AC2 or desaturation injection cells AC1may be smooth or steep. The different cell types may alternate along thesame cell mesa 170 or may be formed in different cell mesas 170. Sincethe function of the meta cells AC3 includes to inject charge carrierseven when the MOS gated channels in the transistor cells are turned on,the meta cells AC3 are formed in a minimum distance of at least 5 μm,e.g., at least 20 μm to the transistor cells TC and at least one of adesaturation injection cell AC1 and a saturation cell AC2 extends fromthe transistor cell TC to the meta cell AC3.

FIGS. 10A to 10C refer to an embodiment with meta cells AC3 directlyadjoining the transistor cells TC. In the meta cells AC3, enhancedsections 125 x of the barrier zones 125 may locally reduce the injectionefficiency of the body zones 115.

Along the contact structures 315 the body zones 115 of the transistorcells TC may include heavily doped contact zones 115 a that improve theohmic contact and overcurrent switching ruggedness in the transistorcells TC. By contrast, the body zones 115 of the saturation anddesaturation injection cells AC1, AC2 may be devoid of heavily dopedcontact zones 115.

In addition, outside the contact zones 115 a the body zones 115 of thetransistor cells TC may have a higher dopant concentration than the bodyzones 115 of the desaturation injection cells AC1, the saturationinjection cells AC2, or both such that emitter efficiency of the bodyzones in the concerned injections cells is lower than in the transistorcells TC.

The RC-IGBT 501 illustrated in FIG. 11 combines a barrier structure 125with cell mesas 170 with bottleneck sections 170 a as described withreference to FIGS. 7A to 7C. In the illustrated embodiment the barrierstructure 125 is uniform. According to other embodiments the barrierstructure 125 may be patterned and may include gaps or attenuatedportions in the desaturation injection cells AC1 as described withreference to FIGS. 5A to 6A. Further embodiments may include meta cellsAC3 as described with reference to FIGS. 9A to 100. Although specificembodiments have been illustrated and described herein, it will beappreciated by those of ordinary skill in the art that a variety ofalternate and/or equivalent implementations may be substituted for thespecific embodiments shown and described without departing from thescope of the present invention. This application is intended to coverany adaptations or variations of the specific embodiments discussedherein. Therefore, it is intended that this invention be limited only bythe claims and the equivalents thereof.

What is claimed is:
 1. A semiconductor device comprising: transistorcells configured to connect a first load electrode with a driftstructure forming first pn junctions with body zones when a gate voltageapplied to a gate electrode exceeds a first threshold voltage; firstauxiliary cells in a vertical projection of and electrically connectedwith the first load electrode and configured to inject charge carriersinto the drift structure at least in a forward biased mode of the firstpn junctions; and second auxiliary cells configured to inject chargecarriers into the drift structure at high emitter efficiency when in theforward biased mode of the first pn junctions the gate voltage is belowa second threshold voltage lower than the first threshold voltage and atlow emitter efficiency when the gate voltage exceeds the secondthreshold voltage.
 2. The semiconductor device according to claim 1,wherein the first auxiliary cells are configured to inject chargecarriers into the drift structure at high emitter efficiency when thegate voltage is below a further threshold voltage between the firstthreshold voltage and the second threshold voltage and at low emitterefficiency when the gate voltage exceeds the further threshold voltage.3. The semiconductor device of claim 1, wherein at a gate voltage belowthe further threshold voltage an increase of the forward voltage dropacross the first auxiliary cells with increasing gate voltage is lesssteep than above the further threshold voltage.
 4. The semiconductordevice of claim 1, wherein at a gate voltage below the second thresholdvoltage an increase of the forward voltage drop across the secondauxiliary cells with increasing gate voltage is less steep than abovethe further threshold voltage.
 5. The semiconductor device of claim 1,wherein at a gate voltage below the second threshold voltage aninjection efficiency at which the second auxiliary cells inject minoritycharge carriers into the drift structure is higher than above the secondthreshold voltage.
 6. The semiconductor device of claim 1, wherein at agate voltage below the second threshold voltage a total injection ofminority charge carriers into the drift structure is higher by thesecond auxiliary cells than by the first auxiliary cells.
 7. Thesemiconductor device of claim 1, further comprising: a pedestal layerbetween the drift structure and a second load electrode, the pedestallayer including first zones and oppositely doped second zones separatingthe first zones, wherein the first zones and the second zones extendfrom the drift structure to the second load electrode, respectively. 8.The semiconductor device of claim 7, wherein the first and second zonesare formed in a bimodal region and the pedestal layer further comprisesa pilot region comprising a pilot zone of the conductivity type of thefirst zones, wherein at least one horizontal dimension of the pilot zoneexceeds at least twice a corresponding horizontal dimension of the firstzones.
 9. The semiconductor device of claim 1, wherein the firstauxiliary cells are evenly distributed.
 10. The semiconductor device ofclaim 8, wherein the first auxiliary cells are arranged in the pilotregion.
 11. The semiconductor device of claim 10, wherein the firstauxiliary cells are arranged in a center of the pilot region.
 12. Thesemiconductor device of claim 10, wherein the first auxiliary cells arearranged in a peripheral portion of the pilot region.
 13. Thesemiconductor device of claim 8, wherein the first auxiliary cells arearranged in the bimodal region.
 14. The semiconductor device of claim 8,wherein the bimodal region surrounds the pilot region.
 15. Thesemiconductor device of claim 13, wherein the first auxiliary cells areformed in the bimodal region in a stripe surrounding the pilot region.16. The semiconductor device of claim 1, wherein the drift structureincludes a drift zone and a barrier structure between the drift zone andthe body zones at least in the second auxiliary cells, and a mean dopantconcentration in the barrier structure is at least ten times as high asa mean dopant concentration in the drift zone.
 17. The semiconductordevice of claim 16, wherein the barrier structure comprises a gap in thefirst auxiliary cells.
 18. The semiconductor device of claim 16, whereinin the first auxiliary cells a mean dopant concentration in attenuatedportions of the barrier structure is at most 50% of a mean dopantconcentration in portions of the barrier structure in the secondauxiliary cells.
 19. The semiconductor device of claim 1, wherein thebody zones are formed in cell mesas between gate structures includingthe gate electrode and extending from a first surface of a semiconductorbody comprising the drift structure and the body zones into the driftstructure.
 20. The semiconductor device of claim 19, wherein cell mesasor portions of cell mesas assigned to the first auxiliary cells arewider than cell mesas or portions of cell mesas assigned to the secondauxiliary cells.
 21. The semiconductor device of claim 19, wherein thecell mesas comprise bottleneck sections and wide sections between thebottleneck sections and the first surface, wherein the wide sections arewider than narrow portions of the bottleneck sections.
 22. Thesemiconductor device of claim 21, wherein the narrow portions of thebottleneck sections of the first auxiliary cells are wider than thenarrow portions of the bottleneck sections of the second auxiliarycells.
 23. The semiconductor device of claim 1, further comprising:third auxiliary cells configured to inject charge carriers into thedrift structure at a high emitter efficiency, when in a forward biasedmode of the first pn junctions the gate voltage exceeds the firstthreshold voltage, wherein the first auxiliary cells are configured toinject charge carriers into the drift structure at a high emitterefficiency when the gate voltage is below a further threshold voltagebetween the first threshold voltage and the second threshold voltage andat a low emitter efficiency when the gate voltage exceeds the furtherthreshold voltage.
 24. The semiconductor device of claim 1, wherein thefirst auxiliary cells are formed in wide cell mesas and the secondauxiliary cells are formed in narrow cell mesas.
 25. A semiconductordevice comprising: a semiconductor body comprising a drift structure andcell mesas formed between gate structures extending from a first surfaceof the semiconductor body into the drift structure, the cell mesascomprising bottleneck sections and wide sections between the bottlenecksections and the first surface, wherein the wide sections are wider thannarrow portions of the bottleneck sections; transistor cells comprisingbody zones forming first pn junctions with the drift structure andsecond pn junctions with source zones; first auxiliary cellselectrically connected in parallel to the transistor cells; and secondauxiliary cells electrically connected in parallel to the transistorcells, wherein the narrow portions of the bottleneck sections in thefirst auxiliary cells are wider than the narrow portions of thebottleneck sections in the second auxiliary cells.